This invention is directed to an apparatus to provide a vaporized reactant for chemical-vapor deposition. More specifically, the invention is directed to an apparatus which utilizes a heated quartz reservoir, a quartz valve and quartz supply lines to provide a vaporized reactant for chemical-vapor deposition.
Numerous semiconductor alloys are being found to be useful in electronic and optoelectronic applications. Group III-V materials comprised of elements such as gallium, arsenic, indium, phosphorous and aluminum are becoming increasing important as adjuncts to silicon already in widespread use in high-speed integrated circuits and optoelectronic emission and detection devices operable in the visible region of the spectrum. Group II-VI materials comprised of elements such as mercury, cadmium, tellurium, zinc, sulphur and selenium are valuable for infrared detection. Compounds using sulphur and selenium are particularly used in blue and ultraviolet optoelectronic applications. It is also possible to combine various elements to produce desired electromagnetic behaviour, including superconductivity.
Various techniques have been developed to deposit such materials in the form of thin films and to grow them epitaxially on suitable substrates. In particular, chemical-vapor deposition is emerging as a useful technique in that it provides more uniform layer thickness and improved surface control when compared to liquid-phase techniques.
In chemical-vapor deposition (also referred to "vapor-phase epitaxy"), all reactants arrive in the gaseous state for deposition on a substrate. In response to the various properties of different chemical elements, a variety of vaporization and delivery techniques have been developed. In some cases, elemental sources may be heated and vaporized or sublimed directly. In other cases, elements may be combined with hydrogen to form a hydride which is a stable gas at room temperature. For example, hydrogen will react with arsenic to form arsine (AsH.sub.3), with phosphorus to form phosphine (PH.sub.3), with sulphur to form hydrogen sulfide (H.sub.2 S) and with selenium to form hydrogen selenide (H.sub.2 Se).
For heavier metals or for elements which do not conveniently form hydrides or other suitable gasses, organometallic compounds, such as the metal alkyls trimethyl-gallium [(CH.sub.3).sub.3 Ga] and trimethylaluminum [(CH.sub.3).sub.3 Al], are utilized in metal-organic chemical-vapor deposition (MOCVD) processes. In MOCVD processes, a carrier gas, such as hydrogen, is bubbled through or passed over organometallic compounds to produce an intermediate chemical vapor that remains stable until a reaction zone or a deposition area is reached.
Such techniques are particularly useful if the developed vapor exhibits a suitably high vapor pressure at or near room temperature. If the vapor pressure is insufficient at room temperature, the delivery apparatus may have to be heated to enhance the vapor pressure. Importantly, all portions of the delivery apparatus which are placed after vaporization of the reactant must be maintained at a temperature sufficient to sustain a vaporized reactant. Any cooler sections of the apparatus may attract and condense the vapor back into a liquid or a solid. For this reason, external heat sources, such as heat lamps or heating tape, may be applied to those portions of the apparatus occurring after vaporization of the reactant.
U.S. Pat. No. 4,568,397 to Hoke et al. discloses a method for growing a Group II-VI epitaxial layer on a substrate. Part of the method disclosed in Hoeke et al. includes heating a Group II metal (such as mercury) to at least 240.degree. C. in a quartz reservoir and directing a flow of a carrier gas (such as hydrogen) over the heated Group II metal. The lines from the quartz reservoir to the substrate deposition area are heated quartz tubes.
U.S. Pat. No. 4,640,221 to Barbee et al. discloses a system for forming a layer of a material on a surface from a gas. The system in Barbee et al. includes a reservoir for heating a material to form a gas, a low pressure reactor for forming the layer and a connecting means between the reservoir and the reactor. Alternatively, the connecting means is heated to a temperature higher than the temperature of the reactor.
U.S. Pat. No. 4,436,674 to McMenamin discloses a chemical vapor delivery system including a container partially filled with the material to be vaporized, a means for controlling the temperature of the material in the container, and a method for providing a continuous uniform mass flow of the vaporized material.
WO 86/006811 recognizes the need for valving which can be used in systems directing high-purity chemicals which are reactive with oxygen or moisture. Such systems are frequently encountered in the manufacture of electronic devices, optical devices and semiconductor devices. WO 86/06811 discloses a valve composed of an inert polymer and having a layer of metal impermeable to oxygen and moisture.
Regardless of recent advancements made in the art of chemical-vapor deposition there is a continuing need for a chemical-vapor deposition apparatus which can be easily purged to facilitate safe loading and unloading of the substrate into the chemical-vapor deposition area. Additionally, it is desirable to be able to quickly shift between delivery of vaporized reactant and delivery of other reactants or only carrier gas to a chemical-vapor deposition area. There is also a continuing need for an apparatus which can provide a reactant reservoir having a substantially constant reactant level.